Narration to accompany video:
This is the first part of a four-part series on ‘How to interpret knee X-rays’.
A lot of patients are given their X-rays and they take them home and they have their report available perhaps, but won’t necessarily know the anatomy or be able to interpret the different planes of the tissues through which the X-rays are passing. So we’ll go through these different steps slowly and you will be able to understand a lot better by the end of this series.
This is a photograph of an X-ray. It’s fairly clear that it’s the knee but the beginner may not be able to understand the concept of how the X-ray beam is shining through the different layers, and so you may see the different parts one layer upon another.
Some structures, such as the muscles and the soft tissues, are ‘radio-lucent’ and the X-rays pass straight through without leaving any image on the film, while other structures such as the the bones are ‘radio-opaque’ and leave a white image on the film.
Here I am outlining the femur bone, which is the upper bone or the thighbone - the larger bone - and you can see from this that it has two rounded ends (which are called condyles).
Below it is the tibia bone, which is more flattened on the top surface. So there are two points of contact really between the bones - the point of contact on the one side and the point of contact on the other side.
Here I am showing the patella, which is revealed simply as a white shadow because it is overlying the femur bone and there is like a double density through the X-ray. Now the patella does not just simply sit there - it is connected to soft tissues, but we will go through that another time.
The little bone that I have not outlined down by the side of the tibia is the fibula bone. It doesn’t actually form part of the joint itself, although it plays an important role in the function of the knee - so I haven’t outlined that.
Here I am just putting into context for you the X-rays. The knee is of course not in isolation - like in the old song “The knee bone’s connected to the thighbone” - and you can see that the femur (the thighbone) goes right up to the hip at the top. Anything affecting the hip, or affecting the ankle or foot, will actually affect the mechanics of the knee for that very reason.
Now I have marked here on the femur bone the white cartilage - gristle or articular cartilage - which provides the slippery layer at the ends of the bones. The cartilage doesn’t actually show well on X-ray, which is why I have marked it here in a different colour. It is X-ray-lucent - the X-rays largely just go through it without leaving any shadow on the film.
This white articular cartilage is present at the end of the femur bone - which is shown by the arrow here - as well as on the top of the tibia and behind the patella. This wider area of articular cartilage on the femur is because the patella glides up and down this central area.
So you can see from this that X-rays have certain limitations. One has to do a lot of interpretation to understand exactly what you are seeing. Therefore, one radiologist may give a slightly different report from another, depending upon his or her skill at interpreting minor subtleties in the shadows and highlights.
Let’s just have a little review of what we have learned so far.
The first point that is really important is that X-rays are really only revealing the bony structures, and not the soft tissues of muscle and skin. You can adjust the X-ray so that you can get a vague outline of the soft structures, and it can be useful on occasions, but at the standard exposures set by the technician you won’t see them.
Equally the cartilage at the ends of the long bones does not show up well, although one may use extrapolation to interpret what might be happening there, as we will discuss later.
We have also learned that in this particular X-ray view the patella appears only as a superimposed faint white ‘shadow’ - you cannot tell much about it. If it was fractured (or broken) right through the middle you would probably be able to see that, but there are better ways of looking at the patella, as we will see in Parts 2 and 3 of this Series.
As I have mentioned, the doctor has to make an interpretation of what is visible on the film. If you look here at the apparently black space between the femur and the tibia, there is clearly a gap between them. The gap on the two sides here is not the same width. On the right of the picture (which is the outer or lateral side of this knee) the gap is wider than on the other side (the medial side). This is probably because there has been some loss of the softer tissues that normally pad out the gap - that is, the articular cartilage at the ends of the long bones and also the shock absorbing meniscus that fits the space between them. The two menisci are wedge-shaped semi-circular cartilagenous structures that help to accommodate the rounded ends of the femur to the flattened top of the tibia.
In this particular patient it is likely that the one meniscus is absent or damaged, possibly together with some deterioration of the articular cartilage, so that the bones have come almost into contact with one another. The doctor may simply report that there is a reduction in the gap between the long bones, and book a different imaging examination, like an MRI scan, to elucidate why. Or he might go further and interpret that as loss of the meniscus tissue.
Another soft tissue set of structures that you don’t see well and which may need further tests are the cruciate ligaments, which are right in the middle of the joint and attached to the two bony spines one can see at the top of the tibia.
So when the doctor orders your X-rays, he will know that a single view like this has limitations and he will invariably order at least two views taken from different viewpoints. In knee orthopaedics, most surgeons would order a minimum of three, and often four, views - all taken at slightly different angles so that you can interpret and put together the picture into the larger whole.
It is a matter of gleaning what you can from what is effectively a very cost effective set of tests.
So this is the end of this part of the series, and in the next parts I will go into detail of some of the other views so that you can understand more fully the issues to do with interpreting knee X-rays. Thank you.
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